Pulsed laser deposition (PLD) is a versatile deposition technique that has been in use for several years. It is based upon the evaporation of a target material by a high power laser, with its subsequent collection on a desired substrate forming a film on the substrate. The PLD method has many advantages including the ease of deposition, and the formation of crystalline films with good adhesion at low temperatures, even as low as room temperature. Another advantage of the PLD technique is the ability to reproduce the stoichiometry of the target in the film, including that of multi-component targets. PLD is desirable for routine deposition at room or higher temperatures providing high quality crystalline thin films.
Different methods have been used to reduce the problem of producing clean, particulate-free thin films of any material of choice using PLD. This method is typically used in superconductor film growth processes and other coating processes for forming high quality thin films. PLD involves laser ablation and evaporation of a target material by a high power laser. The ablated material forms a plume comprising both undesirable large neutral particulates and desirable atoms and ions all of which get deposited on a substrate. The plume extends in a direction outward from the target. Typically, the substrate is held directly in front of the target, at a distance of a few inches. Examples of various pulsed laser deposition methods include those taught by Takano, et. al., U.S. Pat. No. 5,212,151, Schultheiss, et. al. U.S. Pat. No. 5,028,584, Venkatesan, et. al., U.S. Pat. No. 5,015,492, and Owen, U.S. Pat. No. 5,614,114.
However, the PLD technique has one continuing problem that has been extremely difficult to solve, namely the inclusion of undesirable particulates, typically 0.1 to 5.0 .mu.m in size, which disadvantageously limits PLD commercialization. Conventional PLD methods disadvantageously produce about 400 particles cm.sup.2 /.ANG. particle. density. Typically, laser ablation of the target results in the creation of charged and neutral species in many different sizes. Only species of atomic dimensions of the target material are desired to be deposited on the substrate. If large sized particulates form on the substrate, they limit the uniformity of the deposited thin film and its applications. The origin of these particulates is thought to be multifaceted. Factors include protruding surfaces, craters, micro-cracks in the target that are mechanically dislodged due to laser-induced thermal or mechanical shock, rapid expulsion of trapped gas bubbles beneath the target surface during laser irradiation, and the splashing of molten layers of target material. For most applications, the creation of large particulates poses a serious problem. For tribological applications, it is desirable to deposit coatings with very high hardness on precision bearings. These hard coatings can protect the steel surfaces of bearings from wear, and thereby improve the bearing lifetime. This can be extremely valuable in improving the performance of moving mechanical assemblies in a variety of applications such as machinery, pumps etc. PLD is an ideal technique for depositing such hard coatings, however, the incorporation of hard particulates in the coatings can be detrimental to the bearings. Particulates, especially those of a hard material, can be abrasive and destroy the coating, leading to the production of more debris, and eventually to loss of coating adhesion. The loss of the coating adhesion is highly undesirable. If the coated bearing is used in conjunction with a liquid lubricant, the particulates and debris can also impede the smooth flow of lubricant through a bearing assembly, consequently leading to failure.
The inclusion of particulates during PLD poses a big problem when materials for high performance optical, electronic and microelectromechanical systems (MEMS) are deposited. In general, for these applications, stringent constraints exist for surface smoothness, therefore the tolerance to particulate density and size is generally low. In particular, for multilayer device fabrication, the presence of large particulates that get permanently implanted into the film can limit the resolution, size, and functionality of features that are to be fabricated.
A number of attempts have been made to reduce the particulate density in a PLD film. These include the use of an off-axis method where the substrate is positioned parallel to the plume, a velocity filter method to selectively allow particulates of lower mass, and the dual overlapping laser method. Each of these methods has certain shortcomings.
The off-axis method is taught by Cheung et. al. in U.S. Pat. No. 5,411,772. Cheung teaches a deposition configuration which places a substrate parallel to the propagation direction of the plume which is produced when the laser beam impinges upon a target. The substrate disadvantageously extends parallel to the plume limiting film growth. Material deposition on the substrate can only occur from species in the plume that have a significant velocity component perpendicular to the plume propagation direction. This method disadvantageously relies on the presence of a background gas having a pressure range of 0.1 to 0.001 Torr. The role of the background gas is to cause collisions with the ablated species. Lighter particulates such as atoms and ions can be scattered toward the substrate through collisions, while heavy particulates do not experience a significant lateral diffusion and therefore proceed along their original paths without depositing on the substrate. The necessity for random collisions disadvantageously limits the crystallinity of the films produced at low temperatures, and restricts the growth rate seriously.
The velocity filter method is for eliminating large particulates and relies on the fact that large particles tend to have lower velocities than particles of atomic dimensions. The method uses a filter that will only allow species having a predetermined velocity to pass through this filter and deposit on a substrate. This method has been suggested by Akihama, U.S. Pat. No. 5,126,165. Akihama suggested an apparatus and a method for depositing material of a predetermined velocity. Direction selection is made with a plate having an aperture that selects material in the direction of 0.022 steradian with the normal to the target surface. Velocity selection is done using a chopper through which material is allowed to pass only for a predetermined time. In addition, a predetermined dc voltage is applied between the target and substrate to control the spatial and time distributions of charged particulates arriving at the substrate. The velocity method disadvantageously relies upon differing species velocities which may not completely separate desirable ions from undesirable particulates. The velocity filter method has also been used to measure the velocity distribution of micron-size particulates, as well as atomic and ionic species present in the laser ablation plume. Particulate velocities have been found to be on the order of a few hundred meters per second while those of atomic and ionic species were an order of magnitude larger. This suggests that it would be possible to use velocity selection of species in the plume to prevent large particulates from depositing on a substrate. The velocity filter method has also been adapted to use an electronically actuated shutter, placed between the target and substrate. If a velocity filter is to prevent the entire velocity distribution of large particles from reaching the substrate, a significant fraction of atomic size particles will also be filtered out and the growth rate of the film will disadvantageously drop to almost zero.
U.S. Pat. No. 5,660,746 teaches the dual laser method for forming a film with reduced particulate density using a dual-laser deposition process. The preferred embodiment includes the spatial overlap of two laser pulses on a target, with the lasers being of different wavelengths. The first laser irradiates the target surface to form a molten layer, while the second laser generates a plume from the molten layer. The two pulses have a predetermined delay with respect to each other so as to control the ejection and subsequent deposition of particulates on the substrate. The dual laser method disadvantageously requires the use of two lasers having temporal delays when the first CO.sub.2 laser forms a molten layer on the target and the second UV laser vaporizes the molten target.
A PLD magnetic duct method uses a magnetic duct positioned in the deposition chamber to drag ions along the field lines of a weak curved magnetic field which is uniform along the curved ion path. The method teaches to provide magnetic fields sufficient to "magnetize" and affect the direction of flight of electrons in the plasma. The magnetic field strength is disadvantageously not sufficient to magnetize the ions used for thin film formation. The method suggests that when the electrons are magnetized and the ions are not, the ions tend to drift towards the outer wall of the duct, and not be deposited on a substrate. Jordan et al's magnetic duct PLD method teaches the use of magnetic duct to create a uniform magnetic field to guide electron species along the field lines toward the substrate, and disadvantageously does not apply a sufficiently strong force upon the ablated ions to guide them toward the substrate for the creation of quality thin films.
The predominant problem with PLD methods is the creation and deposition of large particulates that impose a limitation on the applications. Despite numerous advantages of the PLD method, its commercialization has been slow, primarily because of the particulate problem, that has been difficult to solve. These and other disadvantages or problems are solved or reduced using the present invention.